Differential signaling, wherein both positive and negative versions of a signal are used to represent values being carried on the signal, is popular due to its high degree of immunity to noise and its large voltage margin. High speed electronic equipment and their interfaces, such as double data rate (DDR) and double data rate two (DDR2) memory interfaces, take advantage of differential signals to help ensure signal integrity. These signals typically have very sharp edges and very short transitions, referred to as signal crossover. Therefore, a typical automated test equipment (ATE) will lack instrumentation that is capable of measuring signal crossovers.
The Nyquist Sampling Theorem states that to accurately represent a signal, it is necessary to sample the signal at twice the signal's frequency. Therefore, one way to capture the signal crossovers is to sample the signal at a sampling rate that is twice the frequency of the signal crossover.
Another commonly used technique to enable the capture of very high frequency signals that is being generated by a device under test (DUT) is to configure the DUT so that it is producing the event that is to be tested at a specified frequency. Then, the ATE can sample the signal being produced by the DUT at a sampling frequency that is slightly different from the frequency of the event's occurrence on the signal. By sampling the signal produced by the DUT for an extended amount of time, the ATE can effectively and accurately capture the signal and the event. While the sampling frequency can either be greater than or less than the frequency of the signal, typically, the sampling frequency is less than the frequency of the signal, and hence the name, undersampling.
One disadvantage of the prior art is that in many cases, the signal crossovers can sometimes last only a few nano-seconds or less. Therefore, to sample the signal at twice this frequency may not be possible. Furthermore, if the sampling at such a sampling rate were possible, the amount of sample data produced may be very large, which could increase the complexity of the test equipment and the testing process.
A second disadvantage of the prior art is that even though the use of undersampling can enable the capture of the signal crossover, it is not capable of determining if both signals in the differential signal are within a specified voltage threshold. The undersampling of the signal can only indicate if one of the two signals (or both signals) in the differential signal is within the specified threshold, it cannot differentiate between both signals being within the specified threshold (a passing case) and only one signal being within the specified threshold (a failing case).
A third disadvantage of the prior art is that the use of undersampling cannot provide additional information about the signal crossover, such as if the signal crossover is occurring in the middle of the voltage swing of the differential signal, an approximate location of the signal crossover in the voltage swing, and so forth.